Photolithography may be used to form patterns in resin layers by using masks. These masks comprise patterns that differ generally from the patterns formed in the resin layer. These differences of geometry are due to the so-called proximity effects. Typically, the photolithography masks are modified beforehand to account for the proximity effects by implementing so-called optical proximity correction (OPC) methods. Moreover, the proximity effects relating to the etching steps can also appear. The photolithography masks are modified, taking these effects into consideration. The term OPC is generally used for any mask modification method that corrects for optical or etching effects.
In a so-called OPC method, photolithography simulations are implemented that make it possible to compare a pattern simulated in a resin layer and the desired pattern. The result of this comparison is used to modify the mask used for the next simulation, or create the photolithography mask after a sufficient number of photolithography simulations.
To simplify the creation of photolithography masks and to reduce the time it takes for this creation, an approach may comprise organizing the pattern hierarchically, by using a plurality of cells. These cells may comprise other cells, and the lower level cells comprise patterns in the form of polygons. It is thus possible to implement a so-called OPC method on a cell and use this result for the creation of the mask that is simplified if the cell appears a number of times in the entire pattern of the mask.
This hierarchy can be produced prior to the implementation of the OPC method, for example, by forming a mask by copying the identical cells. It is nevertheless possible, during the implementation of the OPC method, to consider the mask as a pattern without hierarchy, and to form a new hierarchy more suited to the implementation of the OPC method.
To improve the quality of the mask created from cells, photolithography simulations are implemented that take into account the elements neighboring the cell once implanted in the mask. Better quality OPC results may be obtained. Nevertheless, only the result of the OPC method for the polygons contained in the cell is retained.
It is also possible to re-implement other OPC methods for the polygons situated in the neighborhood of the boundary between two cells when a cell comprises a number of cells on which so-called OPC methods have been implemented. An example of cells of a mask is shown in FIGS. 1a, 1b, 1c and 1d to illustrate the choice of the polygons on which the so-called OPC methods are typically implemented.
A cell CC of a photolithography mask is shown in FIG. 1a. It can be noted that this cell corresponds to the initial design, or “layout” that is to be formed in a resin layer. The cell CC comprises a cell CA and a cell CB that are adjacent. Each of the cells CA and CB comprises a plurality of polygons, the cell CA comprises three polygons PA1, PA2 and PA3, and the cell CB comprises four polygons PB1, PB2 and PB3 and PB4.
Prior OPC methods may have been implemented respectively for the cell CA and the cell CB. It is possible, typically, to implement other OPC methods on certain polygons to obtain a better photolithography mask. For example, the cell CA, which comprises a first group of polygons GA1, is represented in FIG. 1b. The first group of polygons GA1 comprises the polygons PA1 and PA2. The polygons of the first group GA1 do not have any additional neighbors when the cell CA is placed in the cell CC. On the other hand, a second group GA2, comprising the polygon PA3, corresponds to the polygons of the cell CA, which have a neighborhood no longer the same when the cell CA is in the cell CC. In other words, it is possible to retain the result of a prior OPC method for the polygons of the first group GA1, and it is preferable to re-implement an OPC method for the second group GA2 taking into account the neighborhood of this second group GA2 in the cell CC.
Similarly, it is possible to define a first group GB1 comprising polygons PB2, PB3 and PB4, and a second group GB2 comprising the polygon PB1 (FIG. 1c). The polygons which will be used to implement OPC methods for the cell CC are represented in FIG. 1d. A group GCO comprises the polygons for which an OPC method is implemented and for which the result of the OPC method will be retained. Another group of polygons is represented, namely the group GCA of additional polygons used only to implement photolithography simulations and for which the results of the OPC method are not retained.
Before fabricating the mask, a final checking step is generally implemented, in other words a photolithography simulation of the complete mask obtained after all the OPC methods, by eliminating any hierarchy to obtain a pattern comprising only polygons all situated on one and the same level. The elimination of the hierarchy to obtain a pattern comprising only polygons is an operation in which the mask is “flattened,” which makes it possible to implement a “flat” simulation, as appreciated by the person skilled in the art.
During this checking step, errors may occur, and despite the implementation of the OPC methods that take into consideration the neighborhood of the polygons, which varies according to the arrangement of the cells. These errors may be due to a poor processing of the hierarchy, and the errors of this type are commonly referred by the person skilled in the art by the expression “hierarchical bug” or even a “patch boundary bug.” An error should be understood notably to mean undesirable contacts appearing between patterns, undesirable openings, or even pattern sizes or gaps between patterns which are small enough to exceed recommended minimum dimensions.
The appearance of these errors is due to the use of hierarchically organized cells, whereas an OPC method implemented on the whole of the mask makes it possible to prevent the appearance of errors, but is not however applicable because it requires very long computation times and excessively large computer files. Furthermore, when an error is detected during a checking step, the correction of this error is particularly lengthy and costly. Another approach includes locally modifying the mask after the checking step, but this step may be complex to implement, and the repaired part may be difficult to integrate in the photolithography mask.